A screw driven conveyance device, i.e., that which is configured so that a rotation-driven screw shaft is placed along the traveling route of a conveyance carriage, the conveyance carriage is arranged with a follower roller engaged with the screw shaft, and the conveyance carriage is propelled by rotation-driving of the screw shaft is conventionally well known as described in Japanese Published Unexamined Patent Application No. 1997-58463 (hereinafter, referred to as Patent Document 1), etc. Moreover, in this type of conveyance device, when the traveling route length for the conveyance carriage is long, it is necessary that a plurality of screw shafts are concentrically placed in series to match phases of helical vanes of the respective screw shafts, and the respective screw shafts are operably coupled so that they are operably rotated in the same direction at constant speed. Between the respective screw shafts arranged in series, however, bearings at the both ends of each screw shaft and transmission means between the screw shafts need to be placed, and thus, a significantly long air space between the screw shafts is required. It is necessary to configure so that the follower roller on the conveyance carriage side is smoothly transferred to the downstream screw shaft from the upstream screw shaft so that irrespective of the presence of this air space, the conveyance carriage can be reliably traveled continuously rather than interrupting its traveling.
Further, the screw driven conveyance device described in the Patent Document 1 is configured so that the follower roller on the conveyance carriage side is pushed from behind by the helical vanes of the screw shafts in a conveyance carriage traveling direction. In this configuration, the follower roller on the conveyance carriage side is able to float in an axial direction of the screw shaft between the helical vanes of the screw shafts, and thus, there is a possibility that the traveling conveyance carriage swings back and forth by vibration, etc. As means for solving this problem, as described in Japanese Published Unexamined Patent Application No. 2001-106462 (hereinafter, referred to as Patent Document 2), etc., it is also known that helical vanes of screw shafts are rendered a double-blade structure in which a follower roller on a conveyance carriage side is sandwiched from the both sides of an axial direction of the screw shaft.
Further, the screw shafts shown in the Patent Document 1 are configured so that the helical vanes are appended to a single center shaft rod continued across the entire length in a manner so as to continue across the entire length of the center shaft rod. However, when the entire length of the single screw shaft is lengthened as much as possible, it is not practical to manufacture such a screw shaft in a manner that the helical vane continued across the entire length is appended to a long-scale center shaft rod, from the perspective of manufacturing costs, etc. Thus, as described in the Patent Document 2, it is conceivable that screw shaft single bodies in a unit length are connected in an axial direction to configure a screw shaft having a required length.
A conventionally and generally conceivable configuration to solve the foregoing problem is that in which a pair of front and rear follower rollers engaged with a screw shaft at an interval longer than an air space between the identical screw shafts are arranged on the conveyance carriage side and while the front-side follower roller is being moved within the air space between the screw shafts, the conveyance carriage is driven and propelled via the rear-side follower roller. Although it is certain that there is no problem with the foregoing configuration when the conveyance carriage is traveled at constant speed over the entire area of the conveyance carriage traveling route configured by all the screw shafts arranged in series, in other words, when forwarding pitches of the respective screw shafts are the same, when it is attempted to adopt a configuration such that the forwarding pitches of the screw shafts are changed in a specific region, i.e., when it is attempted to adopt a configuration such that the traveling speed of the conveyance carriage can be changed in a specific region while all the screw shafts are rotation-driven at constant speed, the pair of front and rear follower rollers are engaged over the two (front and rear) regions having a different forwarding pitch, and the conveyance carriage cannot be transferred between the regions having a different forwarding pitch. That is, with the configuration such that the pair of front and rear follower rollers engaged with the same screw shaft is utilized, it is not possible to adopt the configuration such that the forwarding pitches of the screw shafts can be changed in a specific region.
Moreover, as described in the Patent Document 2, even when the helical vanes of the screw shaft are rendered the double-blade structure, an appropriate space (play) is essential between the follower roller on the conveyance carriage side and the helical vanes sandwiching that follower roller. Even when a screw shaft is successfully manufactured with high accuracy so that this space is kept to the least required width, it is not possible to completely eliminate the floating (rattling) in the front and rear directions of the traveling conveyance carriage because of the presence of the least-required-width space. Therefore, when the thus configured conveyance device is utilized as a workpiece conveyance device on a coating line along which a workpiece on a conveyance carriage is automatically coated by a coater, there is a risk that spotty coating may result from even a small amount of rattling (due to vibration, etc.) in the back and forth directions in the conveyance carriage (workpiece).
Further, as described in the Patent Document 2, in the method in which a plurality of screw shaft single bodies manufactured so that the helical vanes are merely appended to the center shaft rod of a unit length are connected in the axial direction so as to configure the screw shaft having a required length, the bending strength of all the screw shaft bodies relies on the bending strength of a connection between the screw shaft single bodies. This necessitates skillful arrangement to increase the bending strength of the connection such as increasing a shaft-direction fitting depth of the screw shaft single bodies or by means of a similar technique. Even if the screw shaft single bodies are successfully connected by a connection having a required bending strength, it is practically impossible to lengthen the entire length of the assembled single screw shaft. Thus, along the lengthy conveyance carriage traveling route, a larger number of screw shafts thus configured need to be disposed in series.